An analog/digital converter is used for various electronic equipment. For example, it is also used for an illuminance sensor. The illuminance sensor detects ambient brightness of a display device such as a mobile phone and a television, and adjusts brightness of the display device itself based on a detection result, in order to reduce power consumption.
Japanese Patent Laying-Open No. 2008-42886 (Patent Document 1) relates to an analog/digital converter that has been already proposed by the inventor of the present application, and an illuminance sensor using the same. A photodiode detects ambient light, converts it into a current, and outputs the current to the analog/digital converter. The analog/digital converter integrates the input current, and outputs a digital value in accordance with the level of the light detected by the photodiode.
Further, the illuminance sensor of Patent Document 1 includes a photosensor and a capacitor. While the capacitor is charged with an output current of the photosensor in a predetermined charging period, the capacitor is discharged every time when a charge amount in the capacitor reaches a predetermined charge amount. In response to termination of the charging period, a given current is caused to flow out from the capacitor, and illuminance at a location where the photosensor is installed is obtained based on the number in which the capacitor is discharged and time taken to cause all charges in the capacitor to flow out. Further, the photosensor is configured by connecting two photodiodes having different light reception characteristics (spectral sensitivities) in series, and outputs a current as a difference between photocurrents generated in the two photodiodes.
In addition, Japanese Patent Laying-Open No. 2009-157349 (Patent Document 2) relates to a display device, and FIG. 3 thereof shows a state where an external light sensor element and a position sensor element are arranged. The external light sensor element receives external light incident from a front side of a liquid crystal panel shown in FIG. 1 thereof, and is composed, for example, of a light receiving element including a photodiode. The position sensor is installed on a front side opposite to a back side on which a backlight is installed in FIG. 1 thereof, to detect the position of an object to be detected. The position sensor includes, for example, a photodiode, and is used to detect the position of an object to be detected, such as a finger of a user, a touch pen, or the like.